In cellular communication, the most basic form of “handover” or “handoff” is when a phone call or other communication session in progress is redirected from its current cell, which is called a “source cell” or “serving cell”, to a new cell (called “target cell”). In terrestrial cellular radio networks, the source and the target cells may be served from two different cell sites or from a single cell site. In the latter case, the two cells are usually referred to as two “sectors” on the cell site. Such a handover (HO), in which the source and the target cells are different cells—even if they are on the same cell site—is called an inter-cell handover. The purpose of an inter-cell handover is to maintain the call or other currently ongoing service session as the subscriber is moving out of the area covered by the source cell and entering the area of the target cell.
In the discussion herein, a “cell” and its associated base station such as, for example, an evolved Node B (eNB or eNodeB) may be referred to in an interchangeable manner. For example, a User Equipment (UE) may be interchangeably referred to as being handed over to a target cell or a target eNB, or an HO may be interchangeably referred to as being initiated by a source cell or a source eNB, and so on.
In the context of the handover related discussion herein, it is understood that a UE may be physically present and operating (or registered) within its serving cell or may be currently associated with—i.e., under Radio Frequency (RF) coverage of—or attached to the serving cell in some manner such as, for example, through a prior handover. The UE may now need to be handed over to a target cell if so determined by the serving eNB associated with the UE's source cell and providing RF coverage to the UE within the source cell. A target eNB may provide RF coverage over the target cell and in its vicinity.
As is known, when a UE is mobile, it may start receiving RF signals from a Radio Base Station (RBS, or more simply “BS”) or eNB in a neighboring cell along with the RF signals from its serving cell, especially when the UE is in the vicinity of the neighboring cell. The terms “neighboring cell” and “target cell” are used interchangeably herein to refer to a potential candidate cell considered by the UE's serving cell for the HO of the UE. The HO may be required, for example, when the UE is mobile and the strength of the signals received from the UE's source eNB may no longer satisfy a predefined threshold for sustained attachment of the UE to its source eNB, whereas the strength of the signals received from one or more of the neighboring base stations may be more favorable for sustaining the UE's current service session in the network. In that regard, the neighboring base stations may qualify as “potential target” base stations that the source eNB may consider for the HO of the UE. However, the source eNB first may need to be made aware of these potential target eNBs before it can decide whether to initiate the HO and to which cell. Hence, the source/serving eNB may instruct the UE attached to the serving cell and under operative control of the source eNB to perform certain measurements on the potential target cells. The source eNB may use different policies for triggering the UE to do the measurements and when to report them to the source eNB.
In a Third Generation Partnership Project's (3GPP) Long Term Evolution (LTE) cellular network, when a UE receives RF signals from a potential target cell, the UE may report that target cell's signal measurements, as received by the UE, to the UE's serving cell using a Radio Resource Control (RRC) Measurement Report. The UE's Measurement Report may identify one or more candidate eNBs for handover. The UE may perform measurements on the neighbor cells by measuring their Reference Symbols Received Power (RSRP), Reference Symbols Received Quality (RSRQ), and so on. Based on the Measurement Report received from the UE, the source eNB may make a decision whether to handover the UE to a target eNB or not.
There are multiple HO-triggering or Measurement Report-triggering “events” defined for an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) portion of an LTE cellular network. When the criteria or conditions defined for an “event” are satisfied, the UE will generate and send a Measurement Report to its source eNB to assist the source eNB in HO-related decision-making. Currently, there are eight different “events” defined for E-UTRAN in section 5.5.4 of the 3GPP Technical Specification (TS) 36.331, version 12.2.0 (June 2014), titled “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification (Release 12).” The discussion in the section 5.5.4 of the 3GPP TS 36.331 is incorporated herein by reference in its entirety. Each of these eight “events” has different triggering condition. The discussion herein primarily focuses on the Event A5, which refers to a situation when the signals received by a UE from its serving cell become worse than a first predefined threshold, whereas the signals from a neighbor/target cell become better than a second predefined threshold. Thus, Event A5 is a dual-threshold HO-triggering event. However, as discussed later below, the teachings of the present disclosure are applicable to other triggering “events” as well—dual-threshold or single-threshold.
It is noted here that, for ease of discussion, the terms “HO-triggering event,” “Measurement Report-triggering event,” and “trigger event” are used interchangeably herein to refer to an “event” associated with a network defined signal-quality threshold that triggers a UE to generate a Measurement Report. It is recognized that the UE will also generate periodic measurement reports whenever a defined periodic reporting timer expires and a parameter for a configured report amount is met, but this is not considered an “event” in the present disclosure since no signal-quality threshold is involved. It is further recognized that, strictly speaking, fulfillment of the criteria for such an “event,” by itself, may not necessarily result in triggering of an HO unless certain other conditions are fulfilled as well. Thus, although the terms “HO-triggering event” and “Measurement Report-triggering event” are related, they are not exactly the same. However, for ease of discussion, these terms are sometimes used interchangeably in the relevant literature and, hence, such traditional usage is maintained in the present discussion as well.
FIG. 1 shows an exemplary timeline 10 associated with an HO procedure based on the fulfillment of certain criteria defined for the Event A5. As mentioned earlier, LTE defines multiple HO- or Measurement Report-triggering mechanisms that can be used by a source eNB to trigger a UE to send a measurement report and potentially also trigger an HO procedure for the UE. As noted above, the Event A5 is based on two different thresholds—(i) the first threshold “A5-1” referring to a situation when the Primary Cell's (PCell) signal strength becomes worse than a pre-defined Threshold 1, and (ii) the second threshold “A5-2” referring to a situation when a neighbor cell's signal strength becomes better than a pre-defined Threshold 2. Two exemplary A5-1 and A5-2 thresholds are indicated by reference numerals “12” and “14”, respectively, in FIG. 1. It is noted here that the term “source cell” generally refers to a PCell when there is no Carrier Aggregation (CA) present. However, as discussed in the earlier-mentioned 3GPP TS 36.331, when CA is present, the term “source cell” may include multiple serving cells—one PCell and one or more Secondary Cells (SCells). In any event, for ease of discussion, only the non-CA scenario is assumed in the discussion below. Thus, a “source cell” is treated as the PCell, without any discussion of SCells.
The A5-1 and A5-2 thresholds define “entering conditions” specific for the Event A5. Other “events”—such as Event A3, Event B2, and so on—may have their own event-specific entering condition(s) as discussed later below. When an event-specific “entering condition” is fulfilled, the UE may enter the measurement report triggering phase.
In the discussion herein and in the relevant literature, the term “Ms” is generally used to refer to the UE's measurement result of the source/serving cell, not taking into account any offsets defined for the given “event” in the above-mentioned section 5.5.4 of the 3GPP TS 36.331. Similarly, the term “Mn” is generally used to refer to the UE's measurement result of the neighboring cell, not taking into account any offsets. In FIG. 1, and also in FIGS. 2-3 and 6, the RSRP signal strength plot for Mn is shown as an unbroken line 16, whereas the RSRP signal strength plot for Ms is shown as a broken line 18 having the pattern “-•-•-”.
It is seen from FIG. 1 that the A5-2 “entering condition” is met first, at the timing instance 20 as noted at text block 21. On the other hand, the A5-1 “entering condition” is met later, at the timing instance 22. When the A5-1 condition is met at instance 22, the event-specific “entering conditions” for the Event A5 can be considered to have been met because, as noted at text block 23 and as can be seen from the plots 16, 18 in FIG. 1, both of the A5-1 and A5-2 conditions are met at the timing instance 22. If the Event A5 “entering conditions” can be maintained for a period of time, defined as a Time To Trigger (TTT), the UE will generate a Measurement Report during this TTT period and send the report to its source eNB. An exemplary TTT time interval 25 is shown in FIG. 1 and pointed out by the text block 26. The TTT period 25 is generally configured by the network such as, for example, by the source eNB, with the purpose of making sure that the target cell signal strength can be maintained at an acceptable level—here, above the A5 Threshold-2 or A5-2—before handover occurs.
Based on the Measurement Report, the source eNB may make a decision, using certain parameters not relevant here, whether to handover the UE to the neighbor/target cell reported in the UE's Measurement Report and associated with the signal Mn 16. Upon determining that the UE is to be handed over to the reported target cell, the source eNB may initiate an HO procedure for the UE by sending an HO command to the UE and instruct the UE to perform handover to the neighbor cell associated with the signal Mn 16. In FIG. 1, the timeline 28 indicates receipt of the HO command by the UE as noted at text block 29. After a brief HO interruption period 30, the UE may connect to the target/neighbor cell, which is indicated by the reference numeral “32” in FIG. 1.
However, if the UE fails to complete the HO during the HO interruption period 30, Radio Link Failure (RLF) is considered to have occurred. RLF may occur due to different trigger points such as, for example: (i) When a UE reaches a maximum number of retransmissions allowed to connect to a cell such as, for example, a serving cell; or (ii) When the UE-based T310 timer expires. The T310 timer may be started upon receiving N310 consecutive out-of-sync indications for the UE's source/serving cell from the lower layer; or (iii) When a UE receives random access problem indication from the Medium Access Control (MAC) layer; or (iv) When a UE encounters a handover failure. Additional discussion of factors that can trigger a UE to report an RLF condition may be obtained, for example, from section 5.3.11.3 in the earlier-mentioned 3GPP TS 36.331.
The present disclosure relates to RLF associated with a handover. RLF due to HO process is considered a mobility failure. There are several types of mobility failure cases defined, for example, in section 22.4.2 in the 3GPP TS 36.300, version 12.2.0 (June 2014), titled “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 12).” The discussion in section 22.4.2 of the 3GPP TS 36.300 is incorporated herein by reference in its entirety. These mobility failure cases are: (1) “Too late handover” case: Here, the RLF occurs in the source cell before handover was initiated or completed. The RLF occurs after the UE has stayed for a long period of time in the source cell. In this case, after RLF, the UE attempts to re-establish radio link connection with another cell. (2) “Too early handover” case: Here, the RLF occurs in the target cell shortly after a successful HO from the source cell to the target cell or a handover failure occurs during the HO procedure. In this case, after RLF, the UE attempts to re-establish radio link connection with the source cell. (3) “Handover to wrong cell” case: Here, the RLF occurs in the target cell after completion of the HO procedure. However, after RLF, the UE attempts to re-establish radio link connection with another cell which is neither the target cell nor the source cell.
An RLF may be considered “associated with” or occurring “during” an HO procedure when the RLF occurs shortly prior to, during, or shortly after the HO from a source cell to a target cell. After an RLF during the HO procedure, the UE may attempt to reconnect to the network. After its reconnection to the network, the UE may send an RLF Report to the reconnected cell, which, for ease of discussion herein, is represented by the term “target cell”. The UE may include its RLF Report in a UEInformationResponse message to the target cell as discussed in more detail in sections 5.6.5 and 6.2.2 in the earlier-mentioned 3GPP TS 36.331. The discussion in these sections is incorporated herein by reference in its entirety. The target eNB may then forward the UE's RLF report to the source eNB, which was serving the UE immediately prior to the occurrence of the RLF, via a Radio Link Failure Indication message as discussed in more detail, for example, in section 8.3.9 of 3GPP TS 36.423, version 12.2.0 (June 2014), titled “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 application protocol (X2AP) (Release 12).”
The UE may include one or more of the following information in its RLF Report so as to assist the source cell in adjusting its HO-related parameters with the hope to reduce RLF due to handover: (i) RSRP/RSRQ measurement result (signal strength/quality) from the last serving cell and, optionally, the RSRP/RSRQ measurement result from neighbor cells, which can be from different Radio Access Technologies (RATs); (ii) Cell ID of the failed PCell or source cell; (iii) Cell ID of the re-connected cell; (iv) Geographical location information of the UE when the RLF occurred; and (v) the Time Connection Failure (TCF) period identifying the time interval between the actual starting of the HO procedure and the occurrence of the RLF. For example, the TCF period may be 50 seconds. In other words, a TCF interval represents the time elapsed since the last HO initialization—for example, since the UE's reception of the last HO command including the MobilityControlInfo—until the connection failure or RLF. Additional information that a UE may include in its RLF report to the reconnected/target cell is defined in section 5.3.11.3 of the earlier-mentioned 3GPP TS 36.331, the discussion of that section is incorporated herein by reference in its entirety.
As noted above, when the source cell receives the UE's RLF report, it can try to adjust some of the HO-related parameters—more usually, the event-specific thresholds—with the hope that such adjustment(s) can reduce RLF occurrences. For example, in case of the Event A5, the source cell can try to adjust the threshold values A5-1 and/or A5-2.